Many different standards and modulation schemes exist, but one of the most prevalently used in the world of mobile terminals is the Global System for Mobile Communications (GSM). One of the modulation schemes of the GSM standard is the Enhanced Data Rates for GSM Evolution (EDGE) modulation scheme. The EDGE modulation scheme contains an amplitude modulation component and a phase modulation component. Since there is an amplitude modulation component, the power amplifier of a transmitter operating according to the EDGE modulation scheme must be linear or driven according to a polar modulation scheme.
If a polar modulation scheme is used, a phase modulated signal at the desired radio frequency is provided to the input of the power amplifier and an amplitude modulation component is used to vary the supply voltage provided to the power amplifier. As a result, the power amplifier may operate in saturation and efficiency is greatly improved. Unfortunately, the amplitude modulation component that controls the supply voltage provided to the power amplifier causes unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called Amplitude Modulation to Phase Modulation (AM/PM) distortion, and it degrades the spectral purity of the system and the Error Vector Magnitude (EVM).
In order to compensate for the AM/PM distortion, polar modulation systems may pre-distort the phase modulation component provided to the power amplifier such that the AM/PM distortion at the output of the power amplifier is substantially reduced. As an example, see commonly owned and assigned U.S. Patent Application Publication No. 2003/0215025, entitled AM TO PM CORRECTION SYSTEM FOR POLAR MODULATOR, filed May 16, 2002, which is hereby incorporated by reference in its entirety.
One issue with AM/PM pre-distortion, or compensation, is that the AM/PM distortion is dependent on the Voltage Standing Wave Ratio (VSWR) condition at the antenna of the mobile terminal as well as temperature. Previous techniques used to calibrate the AM/PM pre-distortion, or compensation, of a mobile terminal calibrate the AM/PM pre-distortion for ideal temperature and load conditions. However, the load, and thus VSWR condition, at the antenna of the mobile device may vary depending on various environmental factors such as the user placing the mobile terminal near a metallic object. In addition, the ambient temperature of the environment in which the mobile terminal is operating may also change. When the VSWR condition or temperature changes, so too does the AM/PM distortion of the power amplifier of the mobile terminal. As such, there remains a need for an adaptive process for calibrating AM/PM pre-distortion in a mobile terminal operating according to a polar modulation scheme.